This invention relates to parachute construction, and more particularly to a parachute wherein the inflation/development time of the canopy can be automatically controlled in proportion to the ejection airspeed.
In my above patent application, the inflation time of a multistage parachute is controlled sequentially in proportion with its deployment airspeed by employing a non-elastic pull down vent line that is foreshortened by means of a plurality of spaced breakcords. The breakcords are constructed with varying tensile strengths designed to part sequentially at predetermined airspeeds so as to increase the air-turn-around distance in the canopy, and, therefore, delay opening times as the deployment airspeed increases.
Using non-elastic suspension lines including the pull down vent line is in consonance with current thinking among parachute designers that any elasticity in the line rigging causes high rebound forces which accentuate the end forces.
Although my prior construction accomplishes the purposes of reducing the snatch forces and opening shock forces which occur in all parachute deployments, there are several inherent disadvantages which the present invention overcomes.
As can be seen clearly in FIGS. 3-5 in the drawings of my previously filed patent, use of the breakcords at lower speeds causes the top of the fully inflated hemispherical drag surface to be severely dimpled, which substantial deformation causes instability in movement during descent due to excessive spilling of the trapped air under the canopy. Such instability produces severe oscillations that can approach a magnitude of 40.degree. .+-., causing discomfort to the parachutist, and, more important, possible injury in landing contact with the ground. This same unstable condition can arise in a high speed deployment should one of the breakcords fail to break for any reason.
It has been found, that contrary to accepted parachute design philosophy, the use of a pull down vent line having a predetermined elasticity will prevent excessive canopy deformation at the lower deployment airspeeds and avoid excessive oscillations; will simplify the construction and packaging of the chute; and will provide a smoother transition of the applied forces during deployment.
It is known in the art, as illustrated by the publication to Lisi, Ser. No. 357,045 published May 25, 1943 by the Alien Property Custodian, to fabricate a parachute with a pair of nested suspension lines, the outer suspension lines being made of elastic material. The purpose of this published construction is to vary the speed of the inflated parachute during descent by changing the area of the drag surface. However, in the present invention the purpose of the elastic pull down vent line is to decrease the inflation time of the parachute, and it cannot vary the speed of descent after the canopy is inflated.